This invention relates to surgical procedures for implanting devices that interface with neural tissue.
Surgery within the eye usually is designed to remove damaged or altered tissue. The most common examples include removal of the lens that becomes cloudy with age (i.e., cataract), or removal of the jelly (i.e. vitreous) that fills the back portion of the eye behind the lens. Nevertheless, surgery within the eye has the potential to be of use in procedures to artificially stimulate and activate neural tissue in the eye by implanting prosthetic devices which pass pulses of electrical current through electrodes. For instance, some forms of blindness involve selective loss of the light sensitive transducers of the retina. Other retinal neurons remain viable, however, and potentially may be activated by placement of a prosthetic electrode device on the inner (i.e., toward the vitreous) retinal surface. This placement must be mechanically stable, minimize the distance between the electrodes and the neurons, and avoid undue compression of the neurons.
Vitreous is a jelly-like substance that can largely be extracted with special instruments using conventional procedures. There is, however, a transparent, thin (100 micron) layer of vitreous that is closely applied to the inner retinal surface. This layer cannot be so easily removed. This so-called "cortical" vitreous is a trellis-like scaffolding covering the entire inner surface of the retina. The cortical vitreous is highly sensitive to irritation as caused by pressure, other mechanical stimulation, or inflammation. The cortical vitreous, if subjected to even slight pressure, will contract and pull away from the retina, with potentially disastrous results.
Because the cortical vitreous is transparent, methods of identifying it have relied upon using an intraocular suction cannula with a flexible tip. The cannula can be connected to an aspiration system and the tip of the cannula can be positioned several millimeters above the inner surface of the retina. Suction is applied and if a layer of cortical vitreous is present, the cannula bends because the dense, but transparent, cortical vitreous blocks the proximal region of the cannula. This particular method is described in more detail in Glaser, et al., Ophthalmology, 99 (7) 162-1173 (1992). Nevertheless, this method utilizes significantly higher levels of suction (greater than twice the standard amount) to identify, and/or engage a small area of the cortical vitreous under the cannula. This high level of suction, when applied almost directly to the retinal surface, is potentially dangerous. Pulling on the cortical vitreous with this extreme pressure, or inducing traction by application of heat, can lead to retinal damage, including formation of holes and retinal detachment.
Moreover, the above method facilitates cortical vitreous removal from a small area only. Removal from a wide field requires more aggressive manipulation and suction contact. Unfortunately, the tightly adherent and thin layer of cortical vitreous cannot be removed by currently available vitrectomy units because the suction ports that remove vitreous are recessed 1 mm or more from the end of the instrument. Removal of the cortical vitreous has been attempted using enzymatic digestion of the cortical vitreous but this method is uncontrolled, giving highly variable results. Use of expansile intraocular gas to dissect the cortical vitreous has also been suggested, but further studies have also shown highly variable results (Lincoff, H., Fortschr. Ophthalmol., 81: 95-98, 1984).
Nevertheless, in spite of the difficulties with methods of removing the cortical vitreous, it is just this cortical vitreous which must be removed in order for prosthetic, or other devices to be successfully implanted directly onto the retinal surface. In particular, if a retinal electronic implant is to function without inducing contraction of the cortical vitreous and retinal detachment, the implant needs to be in intimate contact with cells on the surface of a denuded retina, i.e. one from which a cortical vitreous has been removed.
Furthermore, removal of the cortical vitreous may also have a therapeutic effect, particularly in diabetic patients with diabetic retinopathy. Jalkh, A. et al., Arch. Ophthalmol. 100: 432-434, 1982. In this condition, abnormal blood vessels and/or cells grow across the scaffolding of the cortical vitreous, leading to contraction of the cortical vitreous and retinal detachment. Absence of the cortical vitreous, as occurs as a natural phenomenon in older patients, is known to prevent most of the severe complications of the diabetes. Surgery, therefore, designed to remove the cortical vitreous, would seem desirable. Yet despite the availability of methods to remove the cortical vitreous, its removal has not been advocated since the techniques are considered too dangerous for regular application, as evidenced by the lack of support in medical journals for their use.
Unfortunately, even if the cortical vitreous can be removed without noxious effects, by the methods summarized above, excessive manipulation of the underlying retina is also problematic since the retina is an extraordinarily fragile part of the brain, with a consistency of wet tissue paper. In particular, retinal neurons are extremely sensitive to pressure; they will die if even a modest intraocular pressure is maintained for a prolonged period of time. Glaucoma, which is one of the leading causes of blindness in the world, can result from a chronic increase of intraocular pressure of only 10 mm Hg.
The eye also generates significant centrifugal force with saccadic (i.e., jerky, high velocity) eye movements, up to several hundred arc degrees/second, and an implant would tend to slide over the retinal surface if not securely attached. Furthermore, the retina if it is perforated or pulled will itself tend to separate from the underlying retinal pigment epithelium and be rendered functionless. Thus, any method of attaching an object to the retina (i.e., using, for example, tacks) may not be practical, primarily because of the typically high downward pressure that such a device would exert on the retina, which inevitably compromise the retinal neurons.
A reliable method of removing the cortical vitreous has, to our knowledge, never been conclusively demonstrated by histological examination. Moreover, a method of implanting an object securely, but without excessive pressure, onto an inner retinal surface (i.e., epi-retinal) that is free of cortical vitreous has, to our knowledge, not heretofore been demonstrated by in vivo examination.